The present invention relates to an apparatus and a method for marking plastic containers. It is known in the prior art that plastic containers, such as PET bottles for example, are provided with various imprints. For instance, it is known to apply colour prints to plastic bottles, but it is also known to apply stamps to the container in order to mark an expiry date for example. In this case, it is desirable to check whether such imprints have actually been applied to the containers.
A method and an apparatus for applying a laser beam to moving containers is known from EP 0 539 735 A1. In this case, a laser beam is applied to returnable PET bottles in order to check the bottles and/or to read a code on the bottles. However, a check to ascertain whether the code has actually been applied to the bottles is not mentioned in this document.
An inspection machine for plastic bottles is known from EP 295 371 B1. This inspection machine comprises an optoelectric sensor which serves to detect a bottle code marking in the form of raised dots or dashes. However, such detection devices are very complex since they have to detect a specific pattern in a precise manner even at high transport speeds. Furthermore, it is known from the prior art that images of the containers are recorded in each case by means of image recording devices in order to ascertain whether the markings have been provided. However, such image recording devices are also very complex since they have to operate correctly even at high transport speeds.
Moreover, various difficulties arise in the known optical checking methods. Firstly, the contrasts in the image recording are often very weak. In addition, the image recording is often made more difficult due to an undefined background. This undefined background may be formed by air, more or less large-pore foam or a product exhibiting different degrees of colour or opacity. Further problems may arise if both the container and the product are transparent. In addition, external light and a small roughness depth may also cause difficulties during the measurement.
The objection of the present invention is to configure an apparatus for marking plastic containers in such a way that a simple and inexpensive checking of imprints that have been made is possible.